1. Field of the Invention
The present invention relates to a thermal head.
2. Description of the Prior Art
FIG. 1 is a plan view of a thermal head 1 of a typical prior art, and FIG. 2 is a magnified perspective view of the thermal head 1. Referring to these drawings, the prior art is described. Plural heating elements 3 are linearly arranged on an electrically insulating substrate 2, and a heating element line 16 is composed. One end of the common side of the heating element line 16 is commonly connected to a common electrode line 4, and the other common end is connected to plural driving circuit elements 6 through individual electrodes 5 formed on every heating element 3. An electric power is supplied to the driving circuit elements 6 from a power circuit element 8 through a power supply line 7.
The common electrode line 4 is connected to a pair of power circuit elements 8. Part of the power circuit elements 8 and power supply line 7 is mounted on a flexible wiring board 9. A control line 10 for generating heat by selectively energizing the heating elements 3 is connected to each driving circuit element 6. Such substrate 2 is mounted on a cooling board 11 made of, for example, metal material.
In such a thermal head 1, the common electrode lines 4 are made of same material as individual electrodes 5, and are formed simultaneously with the individual electrodes 5, and comprise thin film parts 12 formed in a thickness of several micrometers or less by thin film technology, and thick film parts 13 commonly combined with thin film parts 12 at every heating element 3, stretching over the entire length in the arraying direction of the heating elements 3, having one end connected to the power circuit element 8, and formed by thick film technology such as screen printing. The portion of the thick film parts 13 opposite each heating element 3 is formed in a width of W1, overall length of L1 and film thickness of D1 (20 to 30 .mu.m).
If this thermal head 1 of the type for printing on recording paper of A4 size according to the Japanese Industrial Standard, the length L1 of the thick film part 13 is about 232 mm. In the case of the resistivity of the thick film part 13 is about 5 microohm-cm, the film thickness D1 of the thick film parts 13 is 30 .mu.m and width W1 is 5 mm, the resistance of the middle part of the thermal head 1, that is, of near the position about 116 mm apart from the left end in FIG. 16 of the part opposite the heating elements 3 of the thick film parts 13 is about 39 milliohms. Therefore, when an electric power of 24 V, 25 A is supplied to the common electrode 4 from each power circuit element 8, the voltage drop is EQU 25A.times.0.039 .OMEGA..apprxeq.0.98 V (1)
and hence about 4.1% is lowered. By such voltage drop, unevenness occurs in the heat generation of the heating elements 3 corresponding to the parts at the closer side and the remoter side of the power circuit elements 8 of the thick film parts 13, and an uneven contrast occurs when printed, which leads to lowering of print quality.
Concerning such uneven printing due to voltage drop in the thick film parts 13, it has been confirmed, as far as the thermal head 1 is to perform high picture quality printing of gradation printing of video printer or the like, especially sublimate printing in coloring technology, that the uneven contrast may be eliminated if the voltage drop is about less than 1.5%. According to such characteristic of the thick film parts 13, in order to set the voltage drop about less than 1.5%, it is required to control the resistance of the thick film parts under 15 milliohms. To realize such resistance, however, the width W1 of the thick film parts 13 becomes more than 13 mm.
FIG. 3 is a sectional view from line III--III in the cross section of FIG. 2 for explaining the problem when the width W1 of the thick film parts 13 of the common electrodes 4 is increased. A recording paper 14 to be printed by the thermal head 1 is conveyed in the direction of arrow A1 in FIG. 3, and is cut off at a cutting position P1 remote by a predetermined distance L2 from the downstream side end in the conveying direction A1 of the cooling board 11. Therefore, when the width W1 of the thick film parts 13 increases, the distance L3 between the heating elements 3 and the cutting position P1 becomes longer. This distance L3 corresponds to an upper marginal region 15 of the printed recording paper 14. That is, the recording paper 14 is cut off at the cutting position P1, and has a upper marginal region 15 in the length L3, and is printed from the position opposite to the heating elements 3.
Hence, as the width W1 of the thick film parts 13 becomes larger, the substrate 2 on which the common electrodes 4 is formed increase in size, and the manufacturing cost rises. At the same time, the upper marginal region 15 of the recording paper 14 becomes unnecessarily large, and the running cost increases in this respect.